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  1. #51
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    Re: Evap fin spacing and freezeup



    Quote Originally Posted by desA View Post
    Where does the ice first begin to build up in the coil? For each configuration.
    TBH i'm not sure, I'd have to look into it further, I was hoping you'd answer that for me



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    Re: Evap fin spacing and freezeup

    Evaps are very rarely piped so simply.
    I am one who very rarely uses the evap tp produce the TEV required superheat, So just for ease we will say the refrigerant and the fins are the same temperature, and heat transfer is a constant (knowing that this is not true)
    So ice build up, and this depends upon so many factors, coil depth, evap temp, air temp , humidity, velocity! but generally the majority of the ice is going to form on the inlet face of the coil. So on split fin the mass ice forms on the fins with the greatest gap, whilst still allowing high levels of air flow.

  3. #53
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by mad fridgie View Post
    Evaps are very rarely piped so simply.
    I am one who very rarely uses the evap tp produce the TEV required superheat, So just for ease we will say the refrigerant and the fins are the same temperature, and heat transfer is a constant (knowing that this is not true)
    So ice build up, and this depends upon so many factors, coil depth, evap temp, air temp , humidity, velocity! but generally the majority of the ice is going to form on the inlet face of the coil. So on split fin the mass ice forms on the fins with the greatest gap, whilst still allowing high levels of air flow.
    I was thinking along those lines, coil depth, air velocity etc. Obviously the moisture would freeze quickest at the coldest point but whether the majority would reach that point is the question in my mind as it would depend on how quickly the moisture would condense upon impact with the coil and therefore where in the coil it would deposit. Anyway, I like your idea on the split fin design

  4. #54
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by Bigfreeze View Post
    I was thinking along those lines, coil depth, air velocity etc. Obviously the moisture would freeze quickest at the coldest point but whether the majority would reach that point is the question in my mind as it would depend on how quickly the moisture would condense upon impact with the coil and therefore where in the coil it would deposit. Anyway, I like your idea on the split fin design
    For cross counter flow parallel circuits with negligible pressure drop the earlier circuits after the expansion device, post epi-saturation, are the coldest - they have greatest refrigerant wetting than the pipe run approaching the superheating dry-runs.

    Am I wrong?
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  5. #55
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by DTLarca View Post
    For cross counter flow parallel circuits with negligible pressure drop the earlier circuits after the expansion device, post epi-saturation, are the coldest - they have greatest refrigerant wetting than the pipe run approaching the superheating dry-runs.

    Am I wrong?
    No, you're right. Isn't that what I alluded to when I mentioned the warmest air would hit the warmest refrigerant in the counter cross flow configuration, as the refrigerant flows in from the air off side?

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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by Bigfreeze View Post
    No, you're right. Isn't that what I alluded to when I mentioned the warmest air would hit the warmest refrigerant in the counter cross flow configuration, as the refrigerant flows in from the air off side?
    Yes, we're talking about the same thing - I started reading at only the last few recent posts in the thread. I'm just wondering then how reliable is a statement that the ice starts forming on the coil inlet face - under what conditions is it true and under what conditions is it not true - I was thinking along those lines.
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

  7. #57
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by DTLarca View Post
    Yes, we're talking about the same thing - I started reading at only the last few recent posts in the thread. I'm just wondering then how reliable is a statement that the ice starts forming on the coil inlet face - under what conditions is it true and under what conditions is it not true - I was thinking along those lines.
    I think as was mentioned above that the factors are so innumerous that its very hard to say.
    Fin spacing - Icing would not form as rapidly at the air on contact point and would be drawn further into the coil.
    Air velocity - The faster the air is moving the more likely the moisture will travel into the coil before condensing will take place.
    Coil Depth - The deeper the coil the more your temp drop across your evap, the greater amount of moisture deposited
    Humidity - The higher the humidity, the quicker it will condense and therefore ice closer to the air on side - the converse is also true
    Etc Etc

    Change any one of those factors in your coil design and I'd imagine you'd come up with a vastly different outcome.
    The best solution as I see it is to examine the market and prevailing weather conditions you are planning to sell to - in Mikes case canada for a start - then adapt your coil to suit the prevailing weather conditions.
    So he'll need to prevent snow ingress - Horizontal or angled coil, low humidity, and a short but intense heating season.

  8. #58
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by Bigfreeze View Post
    I think as was mentioned above that the factors are so innumerous that its very hard to say.
    Fin spacing - Icing would not form as rapidly at the air on contact point and would be drawn further into the coil.
    Air velocity - The faster the air is moving the more likely the moisture will travel into the coil before condensing will take place.
    Coil Depth - The deeper the coil the more your temp drop across your evap, the greater amount of moisture deposited
    Humidity - The higher the humidity, the quicker it will condense and therefore ice closer to the air on side - the converse is also true
    Etc Etc

    Change any one of those factors in your coil design and I'd imagine you'd come up with a vastly different outcome.
    The best solution as I see it is to examine the market and prevailing weather conditions you are planning to sell to - in Mikes case canada for a start - then adapt your coil to suit the prevailing weather conditions.
    So he'll need to prevent snow ingress - Horizontal or angled coil, low humidity, and a short but intense heating season.
    Ah okay.

    I just had a browse on the first page of this thread - my settings divide the threads into 50 posts per page. I see Des was already asking the same question.

    And now you've given us an exploded view of the points Mad Fridgie was making/listing.

    I agree with the fin spacing supposition.

    I agree with the velocity supposition - increased velocity = increased bypass factor.

    I agree with the coil depth supposition.

    I'm wondering about the humidity supposition. The higher the humidity the higher the air enthalpy and so the longer it will take to cool and so the further it will go through the coil before dropping its moisture. Seems to me that maybe a higher humidity will simply increase the rate of deposition without adjusting the location of deposition?
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by DTLarca View Post
    Ah okay.

    I just had a browse on the first page of this thread - my settings divide the threads into 50 posts per page. I see Des was already asking the same question.

    And now you've given us an exploded view of the points Mad Fridgie was making/listing.

    I agree with the fin spacing supposition.

    I agree with the velocity supposition - increased velocity = increased bypass factor.

    I agree with the coil depth supposition.

    I'm wondering about the humidity supposition. The higher the humidity the higher the air enthalpy and so the longer it will take to cool and so the further it will go through the coil before dropping its moisture. Seems to me that maybe a higher humidity will simply increase the rate of deposition without adjusting the location of deposition?
    That I don't know. i will leave that to a brighter mind than mine
    I'd imagine, once at this stage, that the only way to be certain of anything would be field testing.

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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by Bigfreeze View Post
    That I don't know. i will leave that to a brighter mind than mine
    I'd imagine, once at this stage, that the only way to be certain of anything would be field testing.
    Indeed, hypothesis, experimentation, modify hypothesis, more experimentation then finally we have a theory or a bunch of competing theories to describe the facts seen during experimentation
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

  11. #61
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by mad fridgie View Post
    Evaps are very rarely piped so simply.
    Not really. Most practical coil lacing will follow a progressive trend through the coil - even though they may follow inverted v, or alternative - to manage oil flow effectively. Some coil makers, however, simply don't have a clue & basically take horizontal coil designs & stand them vertically.

    In general, most - or at least a large portion - of the last coil row ends up devoted to superheating the evaporated fluid.

    I am one who very rarely uses the evap tp produce the TEV required superheat, So just for ease we will say the refrigerant and the fins are the same temperature, and heat transfer is a constant (knowing that this is not true)
    Fins will be at some temp between air local temp & refrigerant inside the tubes - depending on how close to the tube surface you measure.

    So ice build up, and this depends upon so many factors, coil depth, evap temp, air temp , humidity, velocity! but generally the majority of the ice is going to form on the inlet face of the coil. So on split fin the mass ice forms on the fins with the greatest gap, whilst still allowing high levels of air flow.
    Can you explain this further? Why would it be the inlet face of the coil & not the middle, or exit?
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by desA View Post
    Can you explain this further? Why would it be the inlet face of the coil & not the middle, or exit?
    With Heat Pump AC units some manufacturers configure the indoor coil for cross counter flow in cooling which results in cross parallel flow in heating. Others prefer to maximise indoor coil capacities for heating and so they use cross parallel flow configurations for cooling.

    But what would be the difference in icing patterns on outdoor unit cross parallel versus counter cross?

    I use these danfoss video embedded into power point to discuss these concepts with techs.

    From the middle of this video to the end you see the air coil is configured for cross counter in heating mode thus cross parallel in cooling mode...

    http://www.youtube.com/watch?v=8sCzOgNeN_E
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

  13. #63
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    Re: Evap fin spacing and freezeup



    Temp-distance plots for counter-cross & parallel-cross evaporator coils.
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by Bigfreeze View Post
    I was thinking along those lines, coil depth, air velocity etc. Obviously the moisture would freeze quickest at the coldest point but whether the majority would reach that point is the question in my mind as it would depend on how quickly the moisture would condense upon impact with the coil and therefore where in the coil it would deposit.
    Based on the TL diagrams above - where would the first ice begin forming?

    I do suspect, though, that there will be different weather conditions to consider:
    1. Cold air, with moisture;
    2. Snow blown into evap inlet;
    3. Snow falling onto evap discharge.

    The evap/fan coupling & direction of air-flow through the heat-pump will play a part here.
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by DTLarca View Post
    For cross counter flow parallel circuits with negligible pressure drop the earlier circuits after the expansion device, post epi-saturation, are the coldest - they have greatest refrigerant wetting than the pipe run approaching the superheating dry-runs.

    Am I wrong?
    Have a look at this, based on the TL diagrams presented above.
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  16. #66
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    Re: Evap fin spacing and freezeup

    I wrote an article on similar matters ( counter versus parallel) a while back - the images are relevant to this topic though

    http://fridgetech.com/articles/acrne...elflowchiller/


    Counter Flow






    Parallel Flow
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

  17. #67
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    Re: Evap fin spacing and freezeup

    Conditions for my part of Canada, indeed for the part of N.A. where 50 million people reside has winter day time temps of +5 to -10 until Jan 1st and -5 to -20 till March. Yes our winters are colder than Ireland and the UK but it must be -20 before the RH gets below 30%. I am being so general here that the statement might be discounted BUT the dewpoint could be anywhere in the coil at different times so under what conditions do you design for. You cannot design for all.

    That said, I would instinctively had the hottest refrigerant hitting the coldest part of the coil to get the most heat transfer but does that cool off the refrigerant too much to provide resources for appropriate superheat (and promote the ice buildup farther back in the coil)? Looking at this discussion, that does not appear to be what BF has said: warmest refrigerant to warmest part of coil.

    Is this a typical design norm in HP Evap coils?

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    Re: Evap fin spacing and freezeup

    [QUOTE=MikeHolm;218164]Conditions for my part of Canada, indeed for the part of N.A. where 50 million people reside has winter day time temps of +5 to -10 until Jan 1st and -5 to -20 till March.

    What RH% at these temps?

    Yes our winters are colder than Ireland and the UK but it must be -20 before the RH gets below 30%.
    So, -20C ; RH 30%
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by MikeHolm View Post
    Conditions for my part of Canada, indeed for the part of N.A. where 50 million people reside has winter day time temps of +5 to -10 until Jan 1st and -5 to -20 till March. Yes our winters are colder than Ireland and the UK but it must be -20 before the RH gets below 30%. I am being so general here that the statement might be discounted BUT the dewpoint could be anywhere in the coil at different times so under what conditions do you design for. You cannot design for all.

    That said, I would instinctively had the hottest refrigerant hitting the coldest part of the coil to get the most heat transfer but does that cool off the refrigerant too much to provide resources for appropriate superheat (and promote the ice buildup farther back in the coil)? Looking at this discussion, that does not appear to be what BF has said: warmest refrigerant to warmest part of coil.

    Is this a typical design norm in HP Evap coils?
    If your warmest air hit your coldest refrigerant first, it will decrease the air temp further, meaning the coils on the air off side with the warmest refrigerant, will have less temperature differential to the air in order to retrieve energy.
    Think of it from a heating point of view where you have a heat exchanger on a dhw tank. The warmest water will enter the top of the hx and the coldest water the bottom. As the cold water moves up it gains energy from the cools water first and finally the hottest as it leaves the HX which means the water will leave the temp at the temp closest to the temp of the tank and the water re-entering the tank will be at its coldest

    One further thing, even though the RH may seem high at 40-50% at -10 this is still a very small amount of moisture as air at that temp can hold damn all moisture. So while the percentage may seem high the quantity is not. We would have far more problems with humidity here than you will ever have
    Last edited by Bigfreeze; 16-01-2011 at 06:47 PM. Reason: extra comment

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    Re: Evap fin spacing and freezeup

    http://www.refrigeration-engineer.co...1&postcount=35

    A link to the Mitsi heat-pump thread, where I mention the troublesome temperature range just above freezing.
    Last edited by desA; 16-01-2011 at 07:02 PM.
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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by desA View Post
    http://www.refrigeration-engineer.co...1&postcount=35

    A link to the Mitsi heat-pump thread, where I mention the troublesome temperature range just above freezing.
    The 0 - +7 range is probably the worst for frosting. The air can hold a decent amount of moisture while the evap temp doesn't get high enough to avoid icing

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    Re: Evap fin spacing and freezeup

    For this application you have to design a system that either gives you best results for a fixed design condition (its ratings) or one that will work well across a range of conditions (best for the end user), in my opinion these are two completely different units.
    If we look at the working conditions, air on could be from 20C to -20C for heating and 40C to -20C for DHW. So when designing the evap you need to calculate to cover all the working conditions.
    If we look practically at ice formation, we should consider the difference between ice and snow formation.
    Most of these types of units actually reduce airflow by snow formation on the air entering face, whilst we tend to have a more compact/dense ice formation in the interior of the coil (less restriction to air) why is it that snow is formed?

  23. #73
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    Re: Evap fin spacing and freezeup

    I do not use the evap coil to produce superheat, so excluding pressure drops the refrigerant is the same temperature.
    But we should look at pressure drops, if your evap is designed to standard 7Cdb and 6Cwb, for opitium performance the pressure drop is likely to be very slight (SST 1C), but if we look at the same evap when the ambient -20C and when it is this cold humidity is a minor, your SST will be -24C, your pressure drop will increase, for two reasons, a massive reduction in density and massive increase in flash gas (OK this is offset slightly by reduced mass flow)

  24. #74
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    Re: Evap fin spacing and freezeup

    MF, and if (for the above condition) I bring on another smaller evap coil with solenoid??? What happens then?

    Or, size it to 0Cdb and 1C wb...I supposed I will then have pressure issues when ambient is 15C?

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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by MikeHolm View Post
    MF, and if (for the above condition) I bring on another smaller evap coil with solenoid??? What happens then?

    Or, size it to 0Cdb and 1C wb...I supposed I will then have pressure issues when ambient is 15C?
    Mike, what we are talking about really is the number of circuits. for the higher ambient you would have fewer circuits, to keep the velocity and heat transfer up, but for lower temps you would have more circuits to keep the velocity/heat transfer the same and the pressure drop low. So when using the higher number of circuits on high ambients, velocity/heat transfer would be lower, than ideal. It would also be likely that a bit of mal-distribution would occur.
    Also a little bit of oil entrapment may occur, personally i do not think this would effect the reliabilty of the compressor (as it would return at regular periods) Looking at it practically, as far as i can see the high limit for evaporting on compressors on R410a, is quite low, so loosing efficiency of the evap at the higher ambients is not such a bad thing.
    I am no expert on evap design, but know enough to ask the right questions to the evap manufactures, and know enough to know when some try and BS me.
    You can have a interwoven coil, where there is 2 pathways for the refrigerant, each having its own expansion device, one could be isolated at the higher ambient conditions.
    Last edited by mad fridgie; 17-01-2011 at 04:27 AM.

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    Re: Evap fin spacing and freezeup

    I think I will pose that question to a couple of manufacturers when I build up some relationships. Are there any examples of the "interwoven coil" that you know of?

    I am sure there are ways of minimizing oil trapping as well.

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    Re: Evap fin spacing and freezeup

    Quote Originally Posted by MikeHolm View Post
    I think I will pose that question to a couple of manufacturers when I build up some relationships. Are there any examples of the "interwoven coil" that you know of?

    I am sure there are ways of minimizing oil trapping as well.
    I have used interwoven coils many times, most commonly on large de-humidification systems, and generally when i have 2 independent refrigeration circuits.
    Due to the market size in NZ a good proportion of the evaps used (even standard ones) are manufactured to order, so having specials does not really effect the cost. (standard materials)

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